This invention relates to granulators used for or as part of a process for reducing the size of materials to form granules. Granules are widely used in the food, cleaning detergent, mineral processing, agrochemical and pharmaceutical industries. In many applications they offer significant advantages over other types of formulations. For example, they are typically low dusting and also offer advantages of ease of clean up of spills.
Granules may be prepared using a number of methods including agglomeration, spray drying and size reduction.
The term granulator is applied generally to equipment that may be used to produce granules. However, more particularly it is usually used to refer to special equipment specifically designed or modified for producing granules.
In the pharmaceutical industry granulation is a standard step that precedes tabletting. In practice in the pharmaceutical industry, it has been found that it is difficult to obtain accurately metered amounts of finely divided materials into a tablet die, because of the inconsistent flow characteristics of finely divided materials. To overcome this problem the finely divided materials are first converted to granules. Forming tablets from granules also tends to improve the integrity of the tablets. One method of forming granules that can be used is first to form a wet mix of the finely divided materials with water and then to extrude this damp plastic wet mix using an extruder. The extrudate is then dried to form a friable dry extrudate which can then be reduced to the final granule size by cutting or fragmentation. Equipment that can be used for both the extrusion stage and the cutting or fragmentation stage are the Manesty Rotorgran or the Jackson-Crockatt granulators.
The granules thus formed are then fed to the tablet die, enabling tablets of uniform mass to be prepared because of the more accurate metering and flow characteristics of granules. These tablets also exhibit improved integrity.
In the pharmaceutical industry granules may also be prepared by a size enlargement process whereby finely divided materials are agglomerated by tumbling the materials in the presence of a mist of water and optional binders. This process can be carried out using a pan granulator, fluidised bed apparatus or a tumbler. More recently new methods have been proposed for forming drug containing pellets. In U.S. Pat. No. 5,709,885 a method of preparing pellets is proposed that involves aggregation of finely divided materials to form a granulate, extrusion of the granulate into strands and then spheronization of these strands.
Granulation processes suitable for the pharmaceutical industry may also be used in the agrochemical industry to prepare granules containing pesticides or other agrochemical active ingredients. Agrochemical granules are usually used by the farmer by adding the granules to water allowing dissolution/dispersion of the granule to enable application of the active ingredient to target species by spray application. This class of granules is usually referred to as water dispersible granules (WG) but can be regarded as including water soluble granules as well as granules intended for direct application to soil or other loci.
In the agrochemical industry such granules have represented a major formulation breakthrough allowing easier handling, metering and clean-up of spillage.
One method of preparing WG granules is described in Australian Patent Application No. 606,719 where a wet mix is first formed with defined ingredients. The wet mix is then extruded, typically using a Manesty Rotorgran or Jackson-Crockatt type granulator having a woven wire mesh screen or a punched plate screen made out of stainless steel. The aperture size of the screen may be varied but is typically in the range of 0.5-2.0 mm. The damp extrudate that emerges from the screen has a diameter similar to that of the screen aperture size. This damp extrudate is then broken down in size to granules by a tumbling action. The tumbling action may be imparted to the extrudate using a tumbler rotating about its usual horizontal or inclined axis or by using a fluidised bed apparatus. The tumbling action causes a cascading motion resulting in shear mixing as extrudate cascades over other extrudate, resulting in the break down of extrudate to form granules. These granules typically have low compactness and redisperse readily. While WG granules prepared using the above described process have been successfully commercialized, the rate limiting step in the production process has been found to be the extrusion stage.
The Manesty Rotorgran includes a curved grid or screen of aperture size that may be varied but which is typically approximately 1 mm. Mounted above and in close proximity to the surface of this screen are reciprocating blades attached to a rotor assembly. They are arranged and operate so that the edges of the blades sweep in a reciprocating scraping action against the tensioned screen as the rotor assembly rotates backwards and forwards. In operation, material is placed into the Manesty Rotorgran from above to contact the screen and is swept by the blades. When the material is friable the sweeping action of the blades over the screen causes the material to be fractured and broken down in size. This size reduction process continues until the broken down material is small enough to pass through the screen apertures. Accordingly, this apparatus when used with coarse friable material provides a material size reduction process as well as a sizing process. This granulator may also be used for size reduction when the material is deformable or plastic. In this case the sweeping scraping action of the blade causes the plastic material to be compressed and forced through the screen, thus forming extrudate strands of approximately the same diameter as the screen openings. This latter process with plastic materials can also be described as an extrusion process and such a process is regarded as a size reduction process as the plastic material is reduced in size to extrudate. Usually the extrudate is further reduced in size to form granules. This can be done by first drying the extrudate or by size reduction while the extrudate is still damp and plastic.
An alternative granulator to the Manesty Rotorgran is the Jackson-Crockatt granulator. This apparatus operates on a similar principle to the Manesty Rotorgran but instead of using a sweeping blade action over a curved screen uses a reciprocating rotational action applied to the blade so that the blade rotates about a vertical axis at the mid point of the length of the blades. The blades are arranged to be in close proximity to a flat horizontal screen. In the Jackson-Crockatt equipment the effective speed of the blade increases as the distance from the rotational axis increases. In common with the Manesty Rotorgran the material has a sweeping action applied by the blades.
In both the Manesty Rotorgran and the Jackson-Crockatt granulators, in use, pressure is applied between blade edge and the screen. In practice, in combination with the sweeping action, this causes frictional wear on the screen and the blade edge. This is particularly the case when used with damp plastic materials that contain abrasive mineral extenders. This is often the case with agrochemical WG formulations. This pressure requirement also means adjustment to provide uniform pressure across the working area is difficult as the characteristics of the plastic material may change from batch to batch and also with temperature. In the case of the Jackson-Crockatt apparatus with the reciprocating rotating blade, if the diameter of the screen and area of the working surface is increased to increase productivity, the tip speed of the blade is increased which can lead to difficulties in satisfactory operation. For example, it is more difficult to achieve satisfactory alignment of the blade and screen. Furthermore, because of higher tip speeds heat build up can cause problems. Accordingly it is not practical to increase the diameter to the extent desired and as set out above the granulator is usually the rate limiting step in granulation processes. This is usually not a difficulty in the pharmaceutical industry where relatively small amounts of material are required to be processed. However in the agrochemical industry where larger quantities of granules are required to be processed, this can be a major problem. Furthermore, as mentioned above, the screen can wear and become damaged leading to unacceptably coarser extrudate being formed and the manufacturing process having to be interrupted and the screen replaced with obvious production and cost penalties. Other types of granulators have been used and these include Fuji Paudal basket granulators and Hosokawa Schugi Bextruder granulators.
Granulators or pellitisers are known where compaction and formation of granules or pellets takes place primarily in the base plate. For example, in the patent of Payer et al (U.S. Pat. No. 5,393,473) a method of extrusion agglomeration of ultra-high molecular weight polyethylene is disclosed. In the Payer process precompacted powder xe2x80x9cis pushed into the compression channels of the pan . . . xe2x80x9d (column 2, lines 22-24). To achieve agglomeration and melting in the compression channels it is a requirement that the compression ratio which is the ratio of the diameter to length of the compression channel is in the range 1:5 to 1:15 and preferably 1:8 to 1:12 (see column 2, lines 63-68). The compression ratio in the Payer invention is well illustrated in FIG. 1 and Examples 1 to 3 are prepared using this equipment. The compression channel also has an initial tapered portion to aid compaction.
Derwent Abstract 97-04107/05 (Nippon Recycle management KK) discloses apparatus that is similar to that of Payer et al. In particular if one refers to the drawing it is apparent that the xe2x80x9ccompression ratioxe2x80x9d of the die in the base plate is at least 5:1. Furthermore, it appears the lower portion of the die is narrower than the upper portion, again similar to the Payer citation.
This invention provides in one form a granulator comprising a thin perforate screen located within a housing, a roller having a curved convex outer surface and located within the housing wherein the roller is adapted to pivot relative to the perforate screen so that in operation plastic material is able to be forced through the perforate screen by the roller surface as the roller pivots.
Preferably the roller pivots about an axis that is generally parallel to the screen and wherein the roller is further adapted to provide, in operation downward pressure on the screen and, in operation compress plastic material between the curved outer surface of the roller and the screen so that plastic material is forced through the perforations as the roller pivots.
Preferably the roller is further adapted to pivot about an axis generally perpendicular to the screen surface and inboard of the roller so that in operation the curved outer surface of the roller contacts the upper surface of the screen and describes an annular pathway about the inboard axis as the roller pivots about its generally parallel axis when the roller pivots about the inboard generally perpendicular axis.
Preferably the curved outer surface is of generally cylindrical shape.
Preferably the perforate screen is generally flat.
In an alternative form this invention provides a method of preparing granular products comprising forming an extrudable plastic wet mix of active ingredient water and optional fillers, extruding the wet mix using a granulator as described above.
Preferably the granular products are water dispersible granules for agricultural use.
The roller may be in the form of a cylinder that is rotatable about its longitudinal axis. An example of such a roller is a wheel where the circumferential tread surface is generally smooth. The surface may also be textured or indented. The surface of the roller may be made from relatively non deformable materials such as steel or rigid plastic material. Alternatively it may be made from deformable materials such as rubber, plastic, or pneumatic materials. The roller may also be simply an arcuate segment of such a cylinder. It is important for the working of the present invention that the outer surface is curved in a convex form. While a cylinder is a suitable shape for the roller when the screen is flat, the roller could also have a spherical shape provided the screen provided a complementary shape to allow the surfaces to come into close proximity as the roller pivots or rotates. Thus, in the case of a sphere, the surface would need to have a recess of part circular cross-sectional shape to accommodate the sphere. The support structure for the outer surface is not critical and spokes could be used as the support. It is also not essential for the roller to be a cylinder or a segment of a cylinder and alternative curved convex rollers can be used, provided in operation they can compress the material between the outer surface of the roller and the screen surface as the roller pivots. Accordingly an outer surface of a parabolic form could be used. The support structure for the outer surface can be of any convenient form, including the spokes mentioned above.
The purpose and function of the housing is simply to contain the material while it is broken down and a variety of shapes are able to be used. A generally cylindrical housing is convenient and avoids dead spots where material may be accumulated and not processed satisfactorily.
The roller, as well as being pivotable or rotatable about its longitudinal axis, is preferably also pivotable or rotatable about an inboard generally vertical axis inwardly disposed from the roller. While in this description the axes are described as generally horizontal and vertical, it will be appreciated that by appropriate linkages the drive means to the roller could be angled to the horizontal or to the vertical to achieve a similar result and these variations come within the scope of the present invention. Furthermore, it will be appreciated that the terms horizontal and vertical relate to the orientation of the granulator in normal use. However, it is possible for substantial changes in such orientation to occur in practice without effecting the principle of operation of the granulator and such variations come within the scope of the present invention.
In use, material is added to the housing above the screen and the material is compressed between the screen surface and the roller surface, which roller is caused to rotate about its longitudinal axis by frictional contact between the tread of the roller and the screen and/or material between the tread and the screen as the roller pivots or rotates about the generally vertical inboard axis.
Preferably the roller consists of two or more wheels rotatable about a central axis perpendicular to these axes.
The vertical axis of rotation, while preferably central, may be partially offset in which case two or more annular pathways are described which pathways may partially overlap.
A drive shaft does not necessarily need to be located at the vertical axis of rotation and alternative drive mechanisms may be used such that rotation or pivoting occurs about the vertical axis of rotation despite the absence of a drive shaft at this point. Preferred granulators however have drive shafts at this vertical axis. Within the scope of the present invention are wheels or cylinders that have independent drive or rotation means.
In operation downward pressure on the screen surface is applied by the roller by having the roller mounted such that vertical movement is restricted. This is conveniently achieved by having the inboard rotational shaft connected to a gear box or chuck of a drive mechanism. Usually the drive mechanism is via a variable speed electric motor or a hydraulic drive mechanism. Other methods of applying downward pressure may be used such as springs or hydraulic pressure.
The screen may be in the form of a woven wire mesh or a punched plate, or other suitable perforated screen. These are usually made of stainless steel but may be made also from plastic or other metals. When the screen is in the form of a mesh or a thin punched plate a supporting base plate is required to be located below the screen to prevent undue flexing and distortion of the mesh or plate. The supporting plate can take the form of a stainless steel plate with an annular array of holes, typically 10-20 mm in diameter to allow broken down material or extrudate to exit from the granulator. While in preferred embodiments the screen is flat, it is possible for the screen to be substantially curved as in the Manesty Rotorgran. In this case a reciprocating action by the roller would be preferred.
Suitable types of wire mesh screens or punched plates are well known in the engineering field and are generally described in standard handbooks such as Chemical Engineer""s Handbook by Perry. As described above, the aperture size of the screen may be varied but it is typically in the range of 0.5-2.0 mm. in diameter and the diameter determines the approximate diameter of extrudate. As set out in Perry, the maximum thickness of punched plates is determined by the diameter of the holes and for 1 mm. diameter holes is 0.6 mm and for 2 mm. diameter holes is 1.2 mm.
According to Perry, thinner plates than these may be used at a lower cost. In the case of standard woven wire screen used as sieves, when the aperture is 1 mm., the wire thickness is approximately 0.5 mm. When the aperture is 2 mm. the wire thickness is approximately 0.75 mm. Generally, the thickness of the plate or screen is less than double the diameter and more usually less than the diameter. The plate or screen is thin.
The wire diameter can be varied with the same aperture and with increased diameter wire the screens have longer life but have lower through put per unit area. For a diameter of aperture of approximately 0.8 mm., the wire diameter generally available ranges from 0.25 mm to 1.0 mm.
It is also possible to adapt a Fuji Paudal type basket granulator which has a cylindrical screen by replacing the sweeping blades with cylindrical rollers. Similarly a Bextruder can be adapted by appropriate replacement of the blades with rollers. In both these cases the normal operation of the rollers would be in a non reciprocating action.
In certain embodiments of this invention the roller is shaped and/or its mode of operation is adapted so as to provide a mulling action over the working area between the outer surface of the roller and the screen.
A mulling action is a smearing spatulate action.
In the granulator of the present invention this mulling action may be obtained by rotation of the roller about the inboard generally vertical axis provided the roller is appropriately shaped. When the roller is in the form of a wheel or a treaded wheel of uniform diameter across its width the track describes an annular pathway. The inner circumference of the pathway is smaller than that of the outer circumference of the pathway. This difference in pathway distance leads to the smearing spatulate effect which provides particularly effective and efficient operation of the granulator for certain formulations. The smearing or mulling effect results from some forced slippage as the inner portion of the wheel or treaded wheel has the same circumferential movement as the outer portion but is constrained to move over a shorter pathway distance.
The amount of mulling can be increased by increasing the width of the tread on the wheel. It may also be varied by changing the inner and outer diameters of the treaded wheel. If the inner diameter is decreased such that its inner and outer circumferences both match the track distance traveled by both portions of the roller surface there will be no mulling effect. Again this may be desirable for certain formulations. The amount of mulling can be adjusted between these two amounts by appropriate adjustment of the two diameters. The mulling effect can also be increased over that obtained with a uniform cylindrical roller by increasing the diameter of the inner diameter while maintaining the outer diameter the same. The mulling effect can be increased even further by decreasing the outer diameter of the wheel as well as increasing the inner diameter.
As an example of the mulling effect, its quantification and how it may be varied, the following calculations are provided by way of illustration.
In this example the roller is cylindrical and its tread has width 20 mm, inner and outer, diameters of 40 mm and the radius of rotation about its inboard axis is 30 mm to the inner circumference of the tread and 50 mm to the outer circumference of the tread. In this example the pathway distance of the inner circumference of the track is 60 mm. The pathway distance of the outer circumference of the track is 100 mm. The mulling effect can be regarded as the relative percentage of these namely             [                        (                                    100              ⁢              π                                      60              ⁢              π                                )                xc3x97        100            ]        -    100    =      67    ⁢    %  
If the inner diameter of the roller is decreased so that there is a general inwards taper with the inner diameter being 24 mm, and the outer remaining at 40 mm, the mulling effect would be reduced by the relative diameters             [                        (                                    100              ⁢              π                                      60              ⁢              π                                )                xc3x97                  24          40                xc3x97        100            ]        -    100    =      0    ⁢    %  
If the inner diameter is changed to 32 mm while again maintaining the outer at 40 mm, the mulling effect is             [                        (                                    100              ⁢              π                                      60              ⁢              π                                )                xc3x97                  32          40                xc3x97        100            ]        -    100    =      33    ⁢    %  
Conversely if the maximum inward taper from above of 40 mm to 24 mm was reversed to be an outward taper of 40 mm to 24 mm the mulling effect would be             [                        (                                    100              ⁢              π                                      60              ⁢              π                                )                xc3x97                  40          24                xc3x97        100            ]        -    100    =      180    ⁢    %  
When the granulator is used to reduce the size of plastic material it is preferable that the granulator also provides some mixing of the material. We believe with certain formulations that the mulling effect has a beneficial effect on mixing as well as the efficient operation of the granulator. However, usually the materials are intimately mixed before addition to the granulator. In preferred embodiments the amount of material in the pathway of the roller is regulated by means of guides, preferably in the form of plough shaped tynes located ahead of the roller and adapted to control the amount of material directed into the pathway of the roller. This control over the amount of material located ahead of the roller can also be achieved using auxiliary rotating or oscillating blades as optionally used in known basket granulators.
While the granulator may be operated by continuous rotation about the inboard axis it may also be operating using a reciprocating action. For example, a forward pathway may be used for 80% of a circumference and then the direction of travel may be reversed to retrace the pathway. This reciprocating action can lead to greater throughput of plastic material and can avoid caking. While achieving good performance with such reciprocating action some unevenness in wear on the screen can occur because the circumferential pathway is not fully used. This problem is overcome by using a reciprocating action in which the forward and reverse pathways are unequal. For example, if the forward pathway is 80% of a revolution and the reverse pathway is 40% this ensures that in use the full pathway is used and that there is full utilization of the screen.
In practice we have found that the granulator of the present invention often provides substantially increased throughput and in some cases reduced screen wear. One of the reasons for the reduced screen wear compared to the Jackson-Crockatt granulator is that because of the annular tracking or working area, the screen can be supported in a manner so as to provide a relatively stable surface, allowing less flexing and wear of the screen. The action of the roller also reduces drag and friction compared to the shearing scraping action of prior art granulators. It is believed that the rolling action of the roller relative to screen is a more efficient means of forcing plastic material through the screen and can lead to higher throughput of product and less screen wear compared to prior art granulators. In some embodiments tensioning of the wire mesh screen is important for efficient operation. However, the amount of tensioning required for optimum results can be determined by relatively simple experimentation with a particular granulator and formulation.
It will be appreciated that as an alternative to pivoting or rotation about the generally vertical inboard axis, the roller may be rolled by reciprocating lateral movement. An example of such an alternative embodiment is a cylindrical roller that is rolled backwards and forwards across a screen. It will also be appreciated that in this alternative embodiment, the mulling effect could be achieved by having the roller tapering in diameter.
Within in the scope of the present invention is a granulator where the relative movement between the screen and the roller is achieved by rotation or movement of the screen whilst maintaining the roller stationary apart, of course, from rotation or pivoting of the roller.
Materials that can be dry or wet granulated using granulators according to the present invention can be selected from a wide range of materials. Active ingredients may include agrochemicals such as herbicides, insecticides, fungicides and fertilizers. Pharmaceutical active ingredients, food ingredients, detergent ingredients and mineral ingredients may, for example, be used